The Fermi Gamma-ray Space Telescope launched in 2008 has revealed
more than 5000 sources. With a sky density of \(\sim \) 0.1 sources deg\(^{2}\) ,
blazars and particularly BL Lacs dominate the \(\gamma \) -ray sky and constitute
more than 75% of the associated sources in all releases of the Fermi
catalogues ,
,  and
. However, all these catalogues present a
fraction of about one-third of the sources as unassociated or unknown. The
discovery that blazars occupy a narrow region in the WISE IR colour-colour
space, the so-called WISE blazar strip [1]}, [2]}, has been used to search for
blazar-like sources within the positional uncertainty regions of the
unidentified or unassociated \(\gamma \) -ray sources (UGSs) leading to many new blazar candidates
[3]}, [4]}, [5]}, [6]}.
